Neuromotor Basis for Motor Control

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Chapter 4

• Neuromotor Basis for Motor Control

Concept The neuromotor system forms the
foundation for the control of movement
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Introduction

• The Neuromotor System
• Components of the central nervous system (CNS)
  and peripheral nervous system (PNS) involved in
  the control of coordinated movement
• Focus of current chapter is CNS structure and
  function
• Chapter 6 will include PNS related structure and
  function for tactile, visual, and proprioceptive
  sensory systems

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The Neuron

• General Structure see Fig. 4.1
• Cell body
• Contains nucleus
• Dendrites
• Extensions from cell body range from 1 to
  thousands per neuron
• Receive information from other cells
• Axon (also known as a nerve fiber)
• Extension from cell body one per neuron with
  branches (known as collaterals)
• Sends information from neuron

• Neuron Nerve cell
• Basic component of the nervous system
• Range in size from 4 to 100 microns

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Types and Functions of Neurons

• Three Types of Neurons
• 1. Sensory Neurons see Fig. 4.2
• Also known as afferent neurons
• Send information to CNS from sensory receptors
• Unipolar 1 axon no dendrites
• Cell body and most of the axon located in PNS
  only axon central process enters CNS

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Types and Functions of Neurons, contd

• 2. Motor Neurons see Fig. 4.2
• Also known as efferent neurons
• Two types influence voluntary movement
• 1. Alpha motor neurons
• Predominantly in spinal cord axons synapse on
  skeletal muscles
• 2. Gamma motor neurons
• In intrafusal fibers of skeletal muscles

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Types and Functions of Neurons, contd

• 3. Interneurons see Fig. 4.2
• Specialized neurons that originate and terminate
  in the brain or spinal cord
• Function as connections between
• Axons from the brain and synapse on motor
  neurons
• Axons from sensory nerves and the spinal nerves
  ascending to the brain

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Central Nervous System (CNS)

• Two components Brain and spinal cord
• The Brain
• 4 structural components most directly involved in
  the control of voluntary movement
• 1. Cerebrum
• 2. Diencephalon
• 3. Cerebellum
• 4. Brainstem

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Brain Components 1. Cerebrum

• One of two components of forebrain
• Two halves
• Right cerebral hemisphere
• Left cerebral hemisphere
• Covered by cerebral cortex
• Gray tissue 2- to 5-mm thick
• Undulating covering of
• Ridges each is called a gyrus
• Grooves each is called a sulcus
• Cortex motor neurons
• Pyramidal cells
• Nonpyramidal cells

Connected by the corpus callosum
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Cerebral Cortex

• Four lobes
• Frontal
• Parietal
• Occipital
• Temporal
• Sensory cortex
• Posterior to central sulcus
• Receives neuron axons specific to type of sensory
  information

Named according to nearest skull bone
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Cerebral Cortex, contd

• Association areas see Fig. 4.4
• Location
• Adjacent to specific sensory areas of sensory
  cortex
• Function
• To associate information from the several
  different sensory cortex areas
• Allow the interaction between perceptual and
  higher-order cognitive functions
• e.g., selection of the correct response in a
  choice-RT situation
• Possible locations for transition between
  perception and action

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Cerebral Cortex, contd

• Primary motor cortex

• Location Structure
• Frontal lobe just anterior to central sulcus
• Contains motor neurons that send axons to
  skeletal muscles

• Function
• Involved in control of
• Initiation and coordination of movements for fine
  motor skills
• Postural coordination

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Cerebral Cortex, contd

• Premotor area
• Location Anterior to the primary motor cortex
• Functions include
• Organization of movements before they are
  initiated
• Rhythmic coordination during movement
• -- enables transitions between sequential
  movements of a serial motor skill (e.g. keyboard
  typing, piano playing)
• Control of movement based on observation of
  another persons performing a skill

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Cerebral Cortex, contd

• Supplementary motor area (SMA)
• Location Medial surface of frontal lobe adjacent
  to portions of the primary motor cortex
• Functions include involvement in the control of
• Sequential movements
• Preparation and organization of movement

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Cerebral Cortex, contd

• Parietal lobe
• Location
• One of the 4 lobes of the cerebral cortex
• Function
• Involved in the integration of movement
  preparation and execution
• Interacts with the premotor cortex, primary motor
  cortex, and SMA before and during movement

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Subcortical Brain Area Important in Motor Control

• Basal Ganglia
• Buried within cerebral hemispheres
• Consist of 4 large nuclei
• Caudate nucleus
• Putamen
• Substantia nigra
• Globus pallidus
• Function involves control of
• Movement initiation
• Antagonist muscles
• during movement
• Force

- Receive info from cerebral cortex and
brainstem - Send info to brainstem
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Basal Ganglia, contd

• Parkinsons Disease
• Common disease associated with basal ganglia
  dysfunction
• Lack of dopamine production by substantia nigra
• Motor control problems BART
• Bradykinesia (slow movement)
• Akinesia (reduced amount of movement)
• Rigidity of muscles
• Tremor

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Brain Components 2. Diencephalon

• 2nd component of forebrain
• Contains two groups of nuclei
• Thalamus
• Functions
• A type of relay station - receives and integrates
  sensory info from spinal cord and brainstem
  sends info to cerebral cortex
• Important role in control of attention, mood, and
  perception of pain
• Hypothalamus
• Critical center for the control of the endocrine
  system and body homeostasis

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Brain Components 3. Cerebellum

• Location Behind cerebrum and attached to
  brainstem See Fig. 4.3
• Structure includes
• Cortex covering
• Two hemispheres
• White matter under the cortex contains
• Red nucleus Where cerebellums motor neural
  pathways connect to spinal cord
• Oculomotor nucleus

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Brain Components 3. Cerebellum contd

• Functions
• Involved in control of smooth and accurate
  movements
• Clumsy movement results from dysfunction
• Involved in control of eye-hand coordination,
  movement timing, posture
• Serves as a type of movement error detection and
  correction system
• Receives copy of motor neural signals sent from
  motor cortex to muscles (efference copy)
• Involved in learning motor skills

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Brain Components 4. Brainstem

• Location
• Beneath cerebrum connected to spinal cord
• 3 components involved in motor control
• Pons
• Medulla
• Reticular formation

• Functions
• Pons
• Involved in control of various body functions
  (e.g. chewing) and balance
• Medulla
• Regulatory center for internal physiologic
  processes (e.g. breathing)
• Reticular formation
• Integrator of sensory and motor info
• Inhibits / Activates neural signals to skeletal
  muscles

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Spinal Cord

• A complex neural system vitally involved in motor
  control
• Structure See Fig. 4.5
• Gray matter H-shaped central portion
• Consists of cell bodies and axons of neurons
• Two pairs of horns
• Dorsal (posterior) horns Cells transmit sensory
  info
• Ventral (anterior) horns Contains alpha motor
  neurons with axons terminating on skeletal muscle
• Interneurons (Renshaw cells) In ventral horn

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Sensory Neural Pathways

• Several neural tracts (called ascending tracts)
• Pass through spinal cord and brainstem
• Connect to sensory areas of cerebral cortex and
  cerebellum
• 2 tracts to sensory cortex especially important
  for motor control
• Dorsal column
• Anterolateral system
• Tract to cerebellum important for motor control
• Spinocerebellar tract Primary pathway for
  proprioceptive info

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Motor Neural Pathways

• Descending tracts
• Travel from brain through spinal cord
• Pyramidal tracts (corticospinal tracts)
• 60 from motor cortex
• Most fibers cross to other side body
  (decussation) in medulla of brainstem
• Involved in control of fine motor skill
  performance
• Nonpyramidal tracts (brainstem pathways)
• Fibers do not cross to other side of body
• Involved in postural control and control of hand
  and finger flexion extension

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The Motor Unit

• An alpha motor neuron and all the skeletal muscle
  fibers it innervates See Figure 4.6
• When a motor neuron activates (fires) all its
  connected muscle fibers contract
• The ultimate end of the motor neural information

• 200,000 alpha motor neurons in spinal cord
• Number of muscle fibers served by a motor unit
  depends on type of movement associated with the
  muscle
• Fine movements
• e.g. eye muscles 1 fiber / motor unit
• Gross movements
• e.g. posture control many fibers (up to 700)
  / motor unit

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Motor Unit Recruitment

• Amount of force generated by muscle contraction
  depends on number of muscle fibers activated
• To increase force, need more motor units
• Process of increasing number of motor units
  involved recruitment
• Recruitment follows size principle
• Size motor neuron cell body diameter
• Size principle recruit smallest motor units
  first (i.e., weakest force produced) then
  systematically increase size recruited until
  achieve desired force

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From Intent to Action The Neural Control of
Voluntary Movement

• Think about the entire process of deciding to
  perform a skill and actually performing it
• The neural activity involved in this process
  typically follows a hierarchical organization
  pattern
• From higher to lower levels of the neuromuscular
  system
• This process is described conceptually in Figure
  4.7 and Table 4.1

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Neural Control of Voluntary Movement

• 1. Higher centers
• Function - Form complex plans according to
  intent, communicates with the middle level via
  command neurons.
• Structures areas involved with memory and
  emotions, SMA, associations cortex
• 2. Middle level -
• Function converts plans to a number of smaller
  motor programs which determine the pattern of
  neural activation required.
• Structures sensorimotor cortex, cerebellum,
  basal nuclei, brainstem nuclei
• Lowest level
• Function specifies tension of particular
  muscles and angle of joints at specific times
  necessary to carry out programs from middle
  control level
• Structures brainstem or spinal cord

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From Intent to Action Brain Structures
Associated with Movement

• Research by Carson and Kelso (2004)
• Demonstrated there is more involved in
  understanding how we control voluntary
  coordinated movement than knowing which brain
  structures involved in which type of movements
• Cognitive intention is a critical component

• Experiment
• Participants performed finger-flexion movement to
  a metronome
• On the beat (synchronize)
• Between beats (syncopate)
• Task involved exactly the same movement but two
  different cognitive intentions
• fMRI results showed
• Different brain regions active for the two
  movement intentions